1. Field of Invention
The present invention relates to the structure-property relationships between ligands and metal complexes and the efficiency of light emission, and, more specifically, to pyridine-bis(oxazoline) (“pybox”) based ligands, lanthanide metal ion (“Ln (III)”) complexes of pybox, and the use of pybox as a sensitizing moiety.
2. Description of the Related Art
Certain metal ions are of considerable interest due to their luminescent (light emission) characteristics, which arise from f-f transitions, e.g., any element from the lanthanide (“Ln”) series including Eu(III), Tb(III) and Tm(III).
The brightness and unparalleled color purity of the emitted light from Ln(III) ions make these metal ions ideal components of the emitting layers in energy-efficient LEDs, as well as in applications such as fluoroimmunoassays, luminescent tags and sensors. (Bünzli, J.-C. G.; Choppin, G. R., Lanthanide Probes in Life, Chemical and Earth Sciences—Theory and Practice. ed., Elsevier: Amsterdam, 1989, which is hereby incorporated by reference herein in its entirety. All other references cited to herein are hereby incorporated by reference herein in their respective entirety(ies).). Further, in contrast to organic emitters, Ln(III) ion emission has no theoretical limit with respect to its quantum yield. (Baldo, M. A.; O'Brien, D. F.; Thompson, M. E.; Forrest, S. R., Phys. Rev. B-Condensed Matter 1999, 60, (20), 14422-14428).
Ln(III) ion emission arises from intra-4f transitions. Since the 4f electrons are shielded from the ligand field by the 5s and 5p orbitals, the emission bands are largely independent of the coordination environment of the ion. Therefore, the emission bands are very sharp (full width at half maximum around 5 to 10 nm), yielding characteristic pure emission colors (see FIG. 1, illustrating the emission colors of Eu(III), Tb(III), and Tm(III)). However, the luminescence is Laporte (parity)-forbidden and spin-forbidden with low absorption coefficients. This means that population of the excited state of the Ln(III) ion will occur most efficiently by energy transfer from the excited state of a ligand as a sensitizer, or antenna, through a Förster-type mechanism. (Förster, T., Chem. Phys. Lett. 1971, 12, (2), 422-4.) This process is displayed in detail in FIG. 2, which is described in more detail infra.
As seen in FIG. 2, the ligand's singlet state is excited, and through inter-system crossing (ISC) it populates a triplet state. The triplet state can subsequently transfer energy (ET) to a coordinated Ln(III) ion, which will ultimately luminesce. The quantum yield of metal-centered luminescence upon excitation QLLn depends on the efficiency of these individual steps and is summarized in the following equation:QLLn=ηISC×ηET×QLnLn where ηISC is the efficiency of intersystem crossing from the singlet to the triplet state of the ligand, ηET the efficiency of the energy transfer from the triplet state to the Ln(III) ion excited state, and QLnLn is the intrinsic quantum yield of the Ln(III) ion emission upon direct excitation. (Chauvin, A.-S.; Gumy, F.; Imbert, D.; Bünzli, J.-C. G., Spectroscopy Lett. 2004, 37, (5), 517-532.)
For an efficient ISC, a gap of approximately 5000 cm−1 between the singlet and triplet states is required. For an efficient ET, the antenna triplet state must be higher in energy than the 4f excited state by about 2,500 to 4,000 cm−1 (otherwise back transfer (BT) is likely to occur), and the ligand should be directly coordinated to the metal ion. (Klink, S. I.; Hebbink, G. A.; Grave, L.; Oude Alink, P. G. B.; van Veggel, F. C. J. M.; Werts, M. H. V., J. Phys. Chem. A 2002, 106, (15), 3681-3689.) (Reinhard, C.; Güdel, H. U., Inorg. Chem. 2002, 41, (5), 1048-1055.) Non-radiative (NR) deactivation of the 4f excited state through lattice, O—H, C—H or N—H vibrations, as well as ligand fluorescence (F), phosphorescence (P) or NR deactivation can decrease the quantum yield of Ln-centered emission and should be prevented through careful system design.
Numerous ligand designs have been described, from simple 2,6-pyridinedicarboxylic acid, shown to sensitize near-IR emission of Yb(III) (see Reinhard, C.; Güdel, H. U. Inorg. Chem. 2002, 41, 1048-1055), and utilized in standards for the determination of quantum yields of emission (see Chauvin, A.-S.; Gumy, F.; Imbert, D.; Bünzli, J.-C. G. Spectroscopy Lett., 2004, 37, 517-532), to more complex chelating architectures, capable of discriminating between different lanthanide ions and of yielding complexes with high quantum yields of luminescence. (Jensen, T. B.; Scopelliti, R.; Bünzli, J.-C. G. Inorg. Chem. 2006, 45, 7806-7814; Moore, E. G.; Xu, J.; Jocher, C. J.; Werner, E. J.; Raymond, K. N. J. Am. Chem. Soc. 2006, 128, 10648-10649). A chelating architecture, or complex, relates to the binding of two or more atoms of a chelator or chelating agent (i.e., a multidentate ligand—a ligand that is capable of donating two or more pairs of electrons in a complexation reaction to form coordinate bonds) with a metal ion.
Pyridine-2,6-bis(oxazoline) or pybox (see FIG. 3), since its first description in 1989, has been the focus of attention as a ligand for coordination complexes in asymmetric catalysis. (Desimoni, G.; Faita, G.; Quadrelli, P., Chem. Rev. 2003, 103, (8), 3119-3154.) The coordination ability of pybox has been documented. Desimoni and co-workers have isolated a 1:1 complex with La(III), in which the coordination sphere of the metal ion is completed with triflate counter-anions and water molecules. (Desimoni, G.; Faita, G.; Filippone, S.; Mella, M.; Zampori, M. G.; Zema, M., Tetrahedron 2001, 57, (51), 10203-10212.) Aspinall and co-workers isolated 2:1 complexes, which also contain solvent molecules and counter-anions to complete the coordination sphere of the metal ion. (Aspinall, H. C.; Dwyer, J. L. M.; Greeves, N.; Smith, P. M., J. Alloys Compds. 2000, 303-304, 173-177.) (Aspinall, H. C.; Bickley, J. F.; Greeves, N.; Kelly, R. V.; Smith, P. M., Organometallics 2005, 24, (14), 3458-3467.) They also observed that these complexes are stable in solution, since no exchange is seen by NMR between coordinated pybox ligands and free ligand present in excess in solution. (Aspinall, H. C.; Greeves, N., J. Organometal. Chem. 2002, 647, (1-2), 151-157).
Although pybox and its derivatives were extensively utilized by Aspinall and co-workers as well as Desimoni and co-workers (see Aspinall, H. C.; Bickley, J. F.; Greeves, N.; Kelly, R. V.; Smith, P. M. Organometallics 2005, 24, 3458-3467; Aspinall, H. C.; Dwyer, J. L. M.; Greeves, N.; Smith, P. M. J. Alloys Compds. 2000, 303-304, 173-177; Aspinall, H. C.; Greeves, N. J. Organomet. Chem. 2002, 647, 151-157; Desimoni, G.; Faita, G.; Filippone, S.; Mella, M.; Zampori, M. G.; Zema, M. Tetrahedron 2001, 57, 10203-10212;) in lanthanide ion complexes for enantioselective catalysis (see FIG. 4, illustrating previously described pybox complexes) pybox and its derivatives have never been reported as sensitizers for lanthanide luminescence. The pybox ligand is extremely versatile, as it allows straightforward derivatization of the para position of the pyridine ring, as well as of the carbon atoms of the oxazoline ring. (See Aspinall, H. C.; Bickley, J. F.; Greeves, N.; Kelly, R. V.; Smith, P. M. Organometallics 2005, 24, 3458-3467; Aspinall, H. C.; Dwyer, J. L. M.; Greeves, N.; Smith, P. M. J. Alloys Compds. 2000, 303-304, 173-177; Aspinall, H. C.; Greeves, N. J. Organomet. Chem. 2002, 647, 151-157; Desimoni, G.; Faita, G.; Filippone, S.; Mella, M.; Zampori, M. G.; Zema, M. Tetrahedron 2001, 57, 10203-10212; Desimoni, G.; Faita, G.; Quadrelli, P. Chem. Rev. 2003, 103, 3119-3154). As seen in FIG. 3, arrows point out the carbon atoms which can be further derivatized.
Accordingly, very little structural information is known about Ln(III) ion complexes with the pybox ligand, and no description of the pybox ligand's sensitization capabilities has been made, as the research efforts have focused on the catalytic capabilities of the complexes.